Sputtering Deposition: How It Works and Where It's Used

This article is part of the PVD Basics series. Start here or view all articles.

The Short Answer

Sputtering deposition is a PVD method that uses energetic ions to dislodge atoms from a solid target. Those atoms then travel through a vacuum chamber and condense onto a substrate to form a thin film.

Unlike evaporation, which heats material until it converts to vapour, sputtering uses a physical momentum transfer process. This difference provides sputtering with specific advantages: improved adhesion, precise composition control for alloys, and the ability to deposit materials with high melting points.

If you require dense, well-adhering films from nearly any solid material—metal, alloy, ceramic, or semiconductor—sputtering is likely the appropriate option.

Scheme of the sputtering deposition process. Martins, R.M.S.. (2008). In-situ X-Ray diffraction studies during growth of Ni-Ti Shape Memory Alloy films and their complementary ex-situ characterisation.

How Sputtering Works

The term "sputtering" refers to the physical process. The following outlines the sequence of events.

Step 1: Create a plasma. A vacuum chamber is filled with a small amount of inert gas—most commonly argon. A high voltage applied between a target (cathode) and the chamber walls or substrate holder (anode) ionises the argon gas, generating a plasma.

Step 2: Accelerate ions. The positively charged argon ions are attracted to the negatively charged target. The voltage difference accelerates them.

Step 3: Dislodge atoms. When an argon ion strikes the target surface, it transfers its momentum. If the transfer is substantial enough, a target atom is ejected. One incoming ion can displace multiple target atoms through a collision cascade.

Step 4: Transport through vacuum. The ejected target atoms traverse the vacuum chamber. At typical sputtering pressures (a few millitorr), they travel in nearly straight lines with few gas collisions.

Step 5: Condense on substrate. The atoms arrive at the substrate and condense to form a thin film. The energy of the arriving atoms—much higher than in evaporation—facilitates movement on the surface and results in dense, well-adhering films.

That is sputtering. No melting. No evaporation. Just atoms dislodged by ion impact.

A Simple Way to Think About It

Consider a billiard ball (an argon ion) striking a tightly packed arrangement of balls (the target surface). The impact results in several balls being dispersed in different directions. Some of those balls land on the substrate.

Sputtering is momentum transfer, not thermal evaporation. This allows for the sputtering of materials that melt at 3,000°C, as there is no requirement for melting.

Sputtering vs Evaporation: The Key Differences

These two PVD methods are frequently compared. Here are the practical differences.

The higher arrival energy in sputtering is its primary advantage. Energetic atoms can reorganise on the substrate surface, fill voids, and produce denser films with improved adhesion.

The trade-off is complexity. Sputtering requires plasma generation and target management. Evaporation only requires a heated crucible.

Types of Sputtering

Sputtering is not a single technique. It is a family. The PVD Basics series discusses each of these in separate articles.

If you are new to sputtering, commence with magnetron sputtering (discussed separately in this series). It is the most commonly used production method. Then familiarise yourself with DC and RF power supplies, as they will dictate the materials you can deposit.

What Affects Film Quality in Sputtering

Target purity. Lower purity results in increased impurities in your film. For semiconductor applications, you require 99.95% or better. For decorative coatings, 99.9% is acceptable.

Target density. A dense target provides clean sputtering. A porous target can outgas, arc, and generate particles. Avoid purchasing a low-density target for critical applications.

Base pressure. The pressure prior to introducing argon. Higher base pressures indicate more residual water vapour and air in the chamber, which can oxidise your film or lead to contamination. Aim for 10⁻⁶ Torr or better.

Sputtering pressure. The typical range is 2 to 20 mTorr. Lower pressure results in fewer gas collisions and more directional deposition. Higher pressure causes more scattering and can enhance uniformity at the expense of rate.

Substrate bias. Applying a negative bias to the substrate attracts positive ions during deposition. This increases film density and enhances adhesion but may elevate film stress.

Common Applications

Semiconductors. Metal layers (Al, Cu, Ti, Ta) and diffusion barriers (TiN, TaN) are sputtered. Sputtering predominantly dominates front-end and back-end metal deposition in chip production.

Hard drives. The magnetic recording layers and read/write head structures are sputtered. This is among the largest-volume sputtering applications.

Optical coatings. Anti-reflective coatings, mirrors, and filters use sputtering when high density and durability are required.

Decorative coatings. Gold, black, and other coloured finishes on watches, taps, and automotive trim are often sputtered.

Solar cells. Transparent conductive oxides and metal contacts in thin-film solar cells employ sputtering.

Tool coatings. TiN, AlTiN, and CrN coatings on cutting tools can be applied by sputtering, although arc evaporation is also commonly used.

When to Choose Sputtering

Choose sputtering when:

• You require dense, well-adhering films
• Your material has a high melting point (tungsten, tantalum, platinum)
• You require precise composition control in alloys
• Your substrate cannot endure high temperatures
• You require uniform films over large areas

Choose evaporation when:

• You require the highest possible purity (no plasma or target impurities)
• Your material has a low melting point (aluminium, gold, silver)
• You prefer the simplest possible system
• Substrate heating is a significant concern

Limitations to Know

Line-of-sight deposition. Sputtering, similar to evaporation, is line-of-sight. It does not effectively coat the sides of deep holes or complex 3D shapes. For such applications, consider CVD.

Poor target utilisation in planar systems. The racetrack erosion pattern wastes 65-75% of the target. Rotary targets enhance this but come at a higher cost.

Particle generation is a risk. Arcing, target defects, or flaking from shields can generate particles that introduce defects. This is an ongoing challenge in semiconductor sputtering.

Insulators require RF power. One cannot DC sputter an insulator. RF power is necessary, which is slower and requires impedance matching.

The Bottom Line

Sputtering deposition is a versatile, production-established PVD method. It is suitable for nearly any solid material, produces dense films with excellent adhesion, and scales from research to high-volume manufacturing.

The primary limitations are line-of-sight deposition and poor target utilisation in planar systems. For many applications, these are acceptable trade-offs for the film quality and material flexibility that sputtering provides.

If you are choosing between sputtering and evaporation, consider two questions: does your material have a high melting point, and do you need dense films? If the answer to either is yes, sputtering is likely your solution.

Brought to you by Stanford Advanced Materials, a supplier of sputtering targets and evaporation materials.